My Mistake, i just saw you provided the site URL, i checked your site, the issue is that the width of the logo is too big, which is causing it to shrink.Presuming that your menu is not going to have too many items, we can increase the logo space and reduce the menu space. Create a child theme if you havn’t already and copy the header.php file into the child theme, make the following edit
Find the below line
<div class="site-branding col-md-4">
change it to
<div class="site-branding col-md-6">
Find the below line
<nav id="site-navigation" class="main-navigation col-md-8" role="navigation">
change it to
<nav id="site-navigation" class="main-navigation col-md-6" role="navigation">
After making the above change, remove the CSS change you made
.main-navigation {
margin-top: 100px;
}